Hypochlorite bleaching of ground wood



Patented July 25, 1950 HYPOCHLQRITE BLEACHING OF. GROUND WOODv DanielO.Ad'ams and George B. Hughey, (loving,-

ton, Va., assignors to West Virginia Pulp and" PaperCompany, New York;N; Y., a corporation cf Delaware Ne Drawing. Application October16,1944, Serial No. 558,952

Claims. 1

Our present invention relates to improvements in the'bleaching of groundwood, more especially ground oak. and other ground woods such aschestnut, chestnut oak, maple, etc., which contain a substantial amountof tannin and other natural color bodies.

In recent years much study has been given to the bleaching of groundwood. Usually and especially for the manufacture of newsprint, groundwood is not bleached. However, the desirable printing characteristics ofpaper containing a large proportion of ground woodhave led to proposalsfor its use in high grades of paper such as magazine, book paper and thelike. In many instances a base ground wood sheet or one containing asubstantial proportion of ground wood is coated with a suitable coatingcomposition. However, to be successful the ground wood must have areasonably good color of substantial permanence, otherwise only a slightproportion of it may be" used in the furnish; or if used in substantialproportion in the base sheet an undesirably heavy coat must be given itin order to have. a final product of acceptable brightness.

Prior toour discovery the most successful attempts-to bleach groundwood. have been those involving the use of sodium peroxide. We havediscovered, however, that under properly controlled conditions evensuperior results'may be had by hypochlorite bleaching; In applying ourimproved hypochlorite method-to the bleaching ofiground hard woods, asfor example oak and other woods containing appreciable amounts oftannin, it is not practicable to obtain as bright a pulp as with groundspruce, for example. We have discovered, however, that the ground hardwoods-such as oak, chestnut, etc., may be made to yield a bleached pulpof comparable brightness with bleached ground spruce or poplar bysubjecting the ground hard Wood to a preliminary dilute causticextraction whereby to remove certain of the incrustations and colorbodies associated with the fiber, which extraction is directly followed,after Washing, by the hypochlorite bleach.

After the hypochlorite bleach is had, the brightness may be furtherincreased by the use of a moderate excess of an anti-chlor, preferablysulfurous acid.

Our invention will be best understood by ref- 2 erenc-e to the followingdetailed examples "illustrative thereof.

Example I.Without caustic extraction A slurry of ground woodpreparedfrom redoa-k 'containing'15-.2"grams ofoverr dried (0': D.) pulpat 5% consistencywa-s heated to F. To this slurry 0.5 grams'oi NaOH wasadded A volume of calcium hypcchlorite solution containing-1.5 gramsavailable chlorine-was added with agitation, given an initial pH of l0.l. Thereafter during the bleach the" pH was maintained between 10 and10.5 by additions of NaOH solution as required. The bleachwaspermittedto proceedt'o exhaustion of the hypochlorite, which requiredtwenty minutes. As soon as exhaustion had occurred 1.5 gramSSOa wereadded and the mixture allowed to stand ten minutes. The pulp wasthickened on a Buechner funnel and thoroughly washed. This bleachingincreased the brightness of the pulp from 43.1% to"57.0'%' asdetefimined by the General Electric reflection meter using a No. 1 filter.When the hypo'chl'orite was increased to 2.25 grams available chlorine(i.' e., 15% based on the O. D. pulp) the exhaustion period was found tobe sixty minutes and a final brightness of 61.2 resulted, with otherconditions having remained the same.

Example II.With caustic extraction A 1745 gram sample of ground woodprepared from red. oak was" treated with. 35 grams NaOH in sufficientsolution to give a final concentration of .1 gram per liter. Thisresulted in a pulpslurry of approximately 5% consistency (O. D. basis).The temperature was maintained at 77 F; and at. the end of one'hourthepulp slurry was: thickened on a filter: and thoroughly washed.

The pulp from the foregoing.extractionfwas then dispersed in water, andsufilcienti calcium hypochlorite solution to contain 17.5 gramsavailable chlorine (equal to 10%- of the. Weight of the unbleached pulp)wasadded to the slurry. The pH was maintained between 10 andIOLSthroughout the bleach. After the bleach liquor was added, theconsistency of the slurry was 5.0% 0. D. and the temperature F. Thebleach was allowed to proceed until the chlorine was practicallyexhausted, and then 168 grams of S02 were added. After ten minutes thepulp was 3 thickened on a filter and thoroughly washed. The resultingpulp had a brightness of 67.5 as determined with the General Electricreflection meter.

When the strength of the caustic extraction solution was increased to 5grams per liter, a final brightness of 71.8 was obtained, otherconditions having remained substantially the same.

In the caustic extraction step temperature has not been found to becritical. At 65 F. substantially the same results were obtained as at140 F. In general no increased benefits were had by increasing theamount of sodium hydroxide beyond 10% based on the dry weight of thepulp.

In carrying out the hypochlorite bleach, brightening of the pulpoccurred at a pH slightly above 7, with optimum results between 10.0 and11.5. Variation in pulp consistency in the hypochlorite bleaching stagebetween 5 and 10% O. D. was without noticeable effect in the result.However, the brightness of the pulp increased with the concentration ofhypochlorite until 10% to 15% available chlorine on unbleached dry pulpbasis was added. Further increases in the quantity of bleach added werefound to cause a decrease in brightness of resulting pulp. In lieu ofcalcium hypochlorite other compounds yielding hypochlorite ions may beused, e. g., the alkali metal hypochlorites and other alkalinesubstances yielding hydroxyl ions may be used in the extraction step,such as sodium carbonate, trisodium phosphate, sodium sulfide, sodiumsulfite, sodium silicate, etc., as well as the corresponding salts ofthe alkali metals or ammonia.

The addition of sulfur dioxide either as such or in the form ofsulfurous acid, or as a bisulfite or other salt yielding sulfite ion ismade in an amount greater than that needed as an antichlor, and has beenfound to result in a brightening of the pulp by a number of points.Various bleaching aids may be used if desired, as for example magnesiumsulfate, sodium silicate, these salts being added to the bleaching bath.However, we have not thus far found them to be necessary,

excellent results having been obtained without their aid. We claim:

1. The method of hypochlorite bleaching of ground wood of the type richin tannin and color bodies, to yield a ground wood of a brightness atleast equal to that which obtains by the hypochlorite bleaching ofground spruce wood, which comprises first extracting therefrom thetannin and color bodies soluble in dilute aqueous caustic alkalisolution at temperatures under the boiling point thereof, and thenreacting the extracted ground wood with hypochlorite ion in alkalinesolution in an amount not substantially in excess of 15% availablechlorine based on the dry ground wood.

2. The method of hypochlorite bleaching of ground wood of the type richin tannin and color bodies, to yield a ground wood of a brightness atleast equal to that which obtains by the hypochlorite bleaching ofground spruce wood, which comprises first extracting therefrom thetannin and color bodies soluble in dilute aqueous caustic alkalisolution at temperatures under the boiling point thereof by treatingsame with a dilute solution of sodium hydroxide in quantity notsubstantially more than ten per cent. sodium hydroxide based on the dryweight of the pulp, washing the so treated ground wood and then reactingthe extracted ground wood with hypochlorite ion in alkaline solution inan amount not substantially in excess of 15% available chlorine based onthe dry ground wood.

3. The method of hypochlorite bleaching of oak ground wood to yield aground wood of a brightness at least equal to that which obtains by thehypochlorite bleaching of ground spruce wood, which comprises firstextracting therefrom the tannin and color bodies soluble in diluteaqueous 9 hydroxide based on the dry weight of the pulp,

and then reacting the extracted ground wood with hypochlorite ion inalkaline solution in an amount not substantially in excess of 15%available chlorine based on the dry ground wood.

4. The method according to claim 1, in which the reaction withhypochlorite ion is carried out with a pH in excess of 10.

5. The method of hypochlorite bleaching of ground wood of the type richin tannin and color bodies, to yield a ground wood of a brightness atleast equal to that which obtains by the hypochlorite bleaching ofground spruce wood, which comprises first extracting therefrom thetannin and color bodies soluble in dilute aqueous caustic alkalisolution at temperatures under the boiling point thereof, and thenreacting the extracted ground wood with hypochlorite ion in an alkalinesolution in an amount in excess of ten per cent. but not substantiallyin excess of fifteen per cent.

,10 available chlorine based on the dry ground wood.

The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 200,085 Patrick Feb. 5, 1878223,670 Farrell Jan. 20, 1880 263,797 Mitscherlich Sept. 5, 18821,052,675 Loomis Feb. 11, 1913 1,203,511 Embree Oct. 31, 1916 1,539,433Schorger May 26, 1925 1,843,467 Traquair Feb. 2, 1932 2,187,016 CraigJan. 16, 194.0

2,359,891 Brallier Oct. 10, 1944 FOREIGN PATENTS Number Country Date101,475 Great Britain Oct. 11, 1917 296,547 Great Britain Sept. 6, 1928298,333 Great Britain Oct. 11, 1928 OTHER REFERENCES Bard: Efiect ofTannins Tech. Assn.

Papers, Series XXIV, 1941, pages 199-203, page 203 esp. pertinent.

1. THE METHOD OF HYPOCHLORITE BLEACHING OF GROUND WOOD OF THE TYPE RICHIN TANNIN AND COLOR BODIES, TO YIELD A GROUND WOOD OF A BRIGHTNESS ATLEAST EQUAL TO THAT WHICH OBTAINS BY THE HYPOCHLORITE BLEACHING OFGROUND SPRUCE WOOD, WHICH COMPRISES FIRST EXTRACTING THEREFROM THETANNIN AND COLOR BODIES SOLUBLE IN DILUTE AQUEOUS CAUSTIC ALKALISOLUTION AT TEMPERATURES UNDER THE BOILING POINT THEREOF, AND THENREACTING THE EXTRACTED GROUND WOOD WITH HYPOCHLORITE ION IN ALKALINESOLUTION IN AN AMOUNT NOT SUBSTANTIALLY IN EXCESS OF 15% AVAILABLECHLORINE BASED ON THE DRY GROUND WOOD.